1. Field of the Invention
This invention relates to a process for producing isoprene by reacting an alkyl tertiary butyl ether with formaldehyde.
2. Description of the Prior Art
Attempts to synthesize isoprene in one step by reacting isobutene or its precursor such as tertiary butanol or an alkyl tertiary butyl ether with formaldehyde have been made repeatedly and a number of processes have been proposed.
For instance, Japanese Kokai Tokkyo Koho (laid-open patent application) No. 46-6,963 discloses a gaseous phase process using a phosphoric acid-calcium oxide-chromium oxide catalyst. However, this process is far from practicable because the catalyst life is short.
Liquid phase processes using various aqueous acid solutions as the catalysts are disclosed in U.S. Pat. No. 3,890,404, Japanese Patent Publication No. 52-30,483 and Japanese Kokai Tokkyo Koho Nos.57-130,928 and 58-69,824. In U.S. Pat. No. 3,890,404, for instance, it is indicated that when the reaction was conducted at 160.degree. C. for 18 minutes using FeCl.sub.2 as the catalyst, isoprene was formed in a yield of 82% of the formaldehyde basis (Example 32). However, in check experiments performed by the present inventors, the procedures described in Example 32 of U.S. Pat. No. 3,890,404, Example 2 of Japanese Patent Publication No. 52-30,483 and Example 8 of Japanese Kokai Tokkyo Koho No. 57-130,928 gave isoprene yields of about 50% or below, as shown later in Reference Examples. Other examples than the above, when checked, gave nothing but similarly low isoprene yields.
U.S. Pat. No. 4,067,923 discloses that isoprene was formed in an yield exceeding 70% when the reaction was carried out batchwise or in the manner of a piston flow using a sulfanilic acid as the catalyst. However, check experiments by the present inventors revealed that the main product was 4,4-dimethyl-1,3-dioxane and that isoprene was formed in a very small amount (refer to Reference Example 5 to be mentioned later). According to the above cited reference, the reaction is carried out in a closed system at a temperature not lower than the critical point of isobutene. However, such reaction conditions require a high pressure, hence a large cost of equipment.
Furthermore, U.S. Pat. No. 2,350,485 describes various reaction modes for producing conjugated diene by the reaction of a tertiary olefin, a tertiary alcohol or a tertiary halides and an aldehyde. However, the disclosure contains only one example which is concerned with the reaction of 2-methyl-2-butene with formaldehyde in a closed system, and, according to said example, the yield of 2,3-dimethyl-1,3-butadiene is as low as 23%. When the present inventors reacted isobutene with formaldehyde under the same conditions as in said example, the yield of isoprene was similar to that of 2,3-dimethyl-1,3-butadiene just mentioned above.
As mentioned hereinabove, the one-step production of isoprene from isobutene and/or its precursor such as a tertiary butanol or an alkyl tertiary butyl ether and formaldehyde involves various problems to be solved. This is one of the major reasons why the so-called two-step process which involves the production of 4,4-dimethyl-1,3-dioxane as an intermediate has been employed for the commercial production of isoprene.